1. Field of the Invention
The present invention relates to a polishing apparatus which has observing means for observing a surface of a material to be polished and a polishing method of polishing a material to be polished using the polishing apparatus.
2. Related Background Art
In the recent years where progresses have been made in configuration of ultra fine semiconductor devices and sophisticatedly stepped semiconductor devices, chemical-mechanical polishing (CMP) apparatuses are known as a working means for polishing with high precision, SOI substrates, semiconductor wafers made of Si, GeAs, InP and the like, wafers having insulating films or metal films formed on surfaces thereof in processes of manufacturing integrated semiconductor circuits, and substrates for displays.
A CMP apparatus which was used by the inventors before achieving the present invention will be described with reference to FIG. 23. FIG. 23 schematically shows the polishing apparatus which was used by the inventors. before achieving the present invention, wherein a material to be polished (wafer) 100 is held by a holding means 200 for holding a material to be polished in a condition where its surface to be polished faces downward and the material to be polished 100 is polished with a polishing pad 400 which has a diameter larger than that of the material to be polished 100 and is made, for example, of polyurethane. This polishing pad 400 mostly has irregularities on a surface thereof or is porous. In FIG. 23, the material to be polished 100 is turned in a direction indicated by an arrow S by driving means which is not shown in the drawings. Further, the polishing pad 400 is turned in a direction indicated by an arrow T by driving means which is not shown in the drawings. The surface of the material to be polished 100 is kept in contact with the polishing pad 400 and polished by turning both the material to be polished 100 and the polishing pad 400 relatively to each other or either one of these members. At this time, an abrasive material (slurry) is supplied from slurry supply means 600 to a gap between the material to be polished 100 and the polishing pad 400 which are in contact with each other. The slurry is, for example, an alkaline aqueous solution in which fine particles of SiO.sub.2 on the order of microns to submicrons are stably dispersed. In FIG. 23, the slurry is supplied from outside between the material to be polished 100 and the polishing pad 400.
A thickness measuring means 700 aligns (specifies) a location to be measured of the surface of the material to be polished 100, irradiates it with a monochromatic laser and measures the thickness of the material to be polished from a phase deviation of reflected light from the surface to be polished. On the basis of data of a measured thickness value, the CMP apparatus modifies polishing conditions required for obtaining a flat surface which is polished with high precision, for example, a polishing time, and a pressure between the material to be polished 100 and the polishing pad 400 which are in contact with each other, and then polishes once again the surface to be polished.
However, the CMP apparatus described above is incapable of measuring a thickness of a material to be polished, modifying polishing conditions on the basis of a measured results and polishing the material with high precision in a short time since the conventional thickness measuring means requires a long time to align the location at which a thickness is to be measured of the surface of the material to be polished. Further, the CMP apparatus has a low alignment accuracy, thereby being hardly capable of accurately measuring a location at which a thickness is to be measured. Accordingly, obtained thickness values have low reliabilities and are hardly usable as data for modifying polishing conditions.